The present invention relates to a pneumatic tire for passenger cars. More particularly, the invention relates to a pneumatic tire for passenger cars which has a main groove and sub-main grooves having an increased groove width so that the tire can exhibit an improved drainability and yet the uniformity of which is not deteriorated.
When a green tire is vulcanized and molded in a molding die, unvulcanized rubber in the tire's tread portion is pushed aside by groove forming ribs of the die, undergoes fluidization and forms a tread pattern. The tire molding die is formed with a main groove forming rib corresponding to a main groove extending in a tire circumferential direction and sub-groove forming ribs corresponding to sub-grooves extending in a tire widthwise direction. When a green tire is molded in the die, the quantity of rubber removed by the main groove forming rib is greater than that of rubber removed by the sub-groove forming ribs. Therefore, disturbance such as corrugation is likely to occur in a belt layer portion located near groove bottoms, depending on the formation condition of the main groove.
In order to provide a pneumatic tire for passenger cars improved in or relating to the drainability, the inventors of the present invention previously proposed such a tire in which the groove width of the main groove that is provided extending in the tire circumferential direction, in a number of at least one, is set to be greater than its groove depth and, in addition, a plurality of sub-main grooves having a groove width larger than their groove depth alike the main groove or grooves is provided, slantly extending at an angle of 3.degree. to 30.degree. to the tire circumferential direction and to a length greater than a defined contact length of the tire as later to be detailed.
However, in the cases of pneumatic tires for passenger cars, the thickness in their tread portion is relatively limited when compared with heavy-duty pneumatic tires, so that if the groove width of the main groove or sub-main grooves is set to be greater than the groove depth in the cases of passenger-car pneumatic tires as above, such a portion of unvulcanized rubber which should be removed by a main groove forming rib during vulcanization and molding cannot fully flow in a central region in the widthwise direction of the main groove forming rib towards outside of the main groove forming rib but tends to press against an underlying belt layer, whereby there tends to occur an undesirable phenomenon such that the belt layer undergoes corrugation or wave-forming deformation. A tire in which the belt layer undergone such corrugation phenomenon does not have a desirable uniformity and involves the problem of an increase in the radial force variation (RFV) and the radial run-out (RRO).